Space- and Structurally-Efficient Recta-Cylinder Molded Liquid Storage Tanks

ABSTRACT

This invention is an improvement to the shape of liquid storage tanks, in particular to unpressurized molded polymer tanks used for thermal storage in solar systems. Although solar storage tanks represent the first commercial application, this invention is well suited to ambient-temperature water and chemical storage tanks as well. 
     By using a tank with a square footprint instead of a cylindrical tank, storage capacity can be increased by more than 20%, while consuming the same floorspace. For large indoor applications, doorway access amplifies the advantages of a rectangular tank versus cylinders. 
     This invention combines the higher volume of square and rectangular footprint tanks with the structural efficiency of cylinder tanks. The tank walls are formed by alternating vertically-stacked cylinders and rectangular prisms. The cylinders take advantage of hoop strength and also provide a place for external reinforcing bands. Edges and corners may be sharp, chamfered or filleted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application does not cross-reference existing non-provisional utility patent applications. However, it is related to USPTO provisional application 61/460,472.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

STATEMENT REGARDING STATE OF CALIFORNIA SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with State of California support under California Energy Commission grant number PIR-08-012. The Energy Commission has certain rights to this invention.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

[1] Many self-supporting (unburied) liquid storage tank designs, of both metallic and polymeric construction, are available in the prior art. A frequent goal is to maximize the volume of contained liquid while reasonably minimizing both the plan area under the tank and the amount of tank materials, the latter to minimize both costs and dry weight. The general structural problem for such tanks is that the deeper the tank and the liquid contained, the greater the hydraulic forces on the tank walls near the bottom. For many indoor applications, the tank must be able to pass through doorways to reach the installation location. Shape, materials, and fabrication method are important considerations for tank design.

[2] There are four methods commonly used in the production of liquid storage tanks. Steel and stainless steel tanks are fabricated from sheet material that is cut and welded, either automated or by hand. Their resistance to elevated pressures and temperatures makes them popular for domestic water heaters and many commercial and industrial applications. But rising material costs and fabrication expense result in high prices relative to storage capacity. Steel tanks generally last less than 10 years, which can be extended for a few years with glass lining. Stainless steel tanks are well suited to high-purity or corrosive chemical applications, but prices are about double those of similar sized steel tanks. U.S. regulations require that pressurized vessels of 120 gallons or more be individually tested during manufacturing, and wall thickness must increase substantially in larger sizes to maintain the pressure rating. For these reasons and also heavy dry tank weights, larger liquid storage tanks are usually unpressurized.

[3] Although some unpressurized tanks are made from steel, such as those used to store crude oil, most unpressurized tanks are made from polymeric materials due to lower cost and corrosion resistance. The least expensive liquid containers per unit volume are soft, shallow membrane bags, but these are typically not practical in and around buildings where ground area is valuable. Also, their relatively thin, flexible walls risk leakage in long-term applications where they are vulnerable to accidental puncture. Rotationally molded rigid polymer tanks are popular for large volume outdoor water storage in residential or agricultural applications due to their durability and low cost, and some smaller rigid vessels are blow molded.

[4] The fourth type of storage tanks use a rigid structure made from a variety of materials and then lined with a flexible polymer membrane. The flexible liners are cut from roll material such as vinyl and welded using radio frequency or ultrasonic equipment. The structural casings are either custom built or delivered unassembled with the liner. Flexible liner tanks make it possible to install very large indoor tanks without the access doors that a rigid tank would require.

[5] The invention described herein is best suited to rigid molded polymer tanks, which provide the most opportunity for complex tank shapes. Tooling costs may increase, but the impact on unit tank price of increased shape complexity is minimal. However, there is no practical limitation preventing the incorporation of this invention into fabricated steel vessels or flexible polymer tanks.

[6] This invention is ideally suited to thermal storage tanks, such as those used in solar systems. Maintaining thermal stratification in a solar storage tank improves system efficiency. Stratification can be promoted and maintained by extracting liquid from the bottom of the tank where it is coolest, and returning it to the top. In polymeric tanks, which have very modest vertical heat conduction through the tank walls, thermal stratification of 20 to 40 degrees C. can be maintained if liquid velocities entering and leaving the tank are limited to prevent mixing. However, the use of polymer materials in thermal storage tanks is particularly challenging because the elevated temperatures can lead to a significant reduction in stiffness and strength.

[7] Liquid storage tanks that more effectively use vertical space are most efficient structurally when configured as vertical-axis cylinders. These “hoop design” tanks can minimize wall thickness using materials with high tensile strength. Rotationally-molded cylindrical tanks are now very common, even for sizes as large as 15,000 gallons. The vast majority of pressurized steel tanks are cylinders to take advantage of the hoop strength principle. In an optimal design, walls of cylindrical tanks are gradually thinner proceeding from bottom to top, matching the hydraulic forces, and some manufacturers of rotational molding equipment are now able to control the process toward this end. While most of these designs are cylindrical, the rotational molding process offers significant opportunities for forming polymeric tanks with complex shapes, a direction pursued with this invention.

[8] The primary disadvantage of the vertical-axis cylinder is that it leaves more than 21% (100*(1−π/4)) of the available volume unoccupied. Where large indoor thermal storage capacity is required, multiple tanks must be used, with each tank able to pass through doorways to gain access to the final location. Again, vertical-axis cylinder tanks provide 21% less capacity than a tank with a square footprint that is able to pass through the same width doorway. A tank with a rectangular footprint extends this advantage further. Square and rectangular tanks make better use of floorspace and require fewer manifold connections. Because they can be installed tight against each other, heat losses and structural requirements for tanks at the center of the array are reduced.

[9] A patent search of six key class/subclass categories yielded not one patent relevant enough to cite. But several relevant rotationally-molded 400 gallon human-height semi-rectangular tanks, labeled “doorway tanks” are available. (See Norwesco http://www.tank-depot.com/productdetails.aspx?part=TN400XWT and Snyder http://www.tank-depot.com/productdetails.aspx?part=Sll-Closet400.) Their goal is to maximize volume in a tank that can fit through a typical residential doorway. Both designs use multiple (two) full perimeter horizontal indentations as wall stiffeners, and both include a pair of horizontal-axis cylindrical holes through the long wall of the rectangular tank. The Norwesco tank places the holes one-above the other, each centered on a horizontal indentation, while the Snyder tank has them side-by-side, between the two horizontal indentations. Neither design maximizes volume due to their large horizontal indentations and the through-cylinders. Both “single-tank” designs contain 10-13% more liquid compared to a pair of side-by-side cylindrical tanks occupying the same floor area. While these available tanks offer cost advantages compared to simple vertical-axis cylinders, further cost-effective volume increases are possible with the design documented here.

BRIEF SUMMARY OF THE INVENTION

[10] The invention describes molded tanks that combine the strength of cylinders with the space economy of rectangular solids. A simple version forms a square in plan view that transitions to a horizontal circle in a plane above its floor, transitioning back and forth from square to circle several times, finishing as a square at the top of the tank. A circular opening may be provided on the top if necessary for access to the interior of the tank. The tank transitions from its predominantly square cross-section to cylindrical bands whose hoop strength prevents significant outward bulging of the side walls. The cylindrical bands are more closely spaced near the bottom where hydraulic pressure is greatest. Spacing of the cylinders depends on wall thickness for the rectangular elements. The walls of these elements span vertically from the floor plane to the first cylinder, then from cylinder to cylinder, and finally, at the top, from cylinder to horizontal top plane. In stratified polymeric thermal storage tanks, higher temperatures at the upper elements may necessitate uniform cylinder spacing, or even closer spacings at the top of the tank, since the polymers have lower strength at higher temperature. The cylinders also provide a convenient location for the placement of reinforcing bands made from high-tensile strength materials such as steel. Such metal bands offer good strength at elevated temperatures, as well as resistance to structural creep, both problematic for polymer materials.

[11] The rotational molding process also allows single tanks that are rectangular in plan with the length of the long side of the rectangle an integer multiple of the length of the short side of the rectangle. In these designs the cylindrical bands are centered in adjacent squares aligned vertically to create through-holes so that the bands provide continuous hoop strength in each square of the multi-square plan. For additional strength, cylinder sections with the horizontal axes can be located where the squares join.

[12] Objects of the invention:

-   -   1. To form a tank that maximizes the volume of liquid contained         upon a rectangular floor area while utilizing hoop strength         principles to minimize the volume of tank wall material needed         to withstand internal hydraulic stresses.     -   2. To form an economical tank with maximum liquid volume that         can fit along a vertical building wall with minimal projection         outward from the wall.     -   3. To form molded polymeric tanks with stacked rectangular and         short cylindrical horizontal cross-sections in which the         cylinders can be reinforced with external high-tensile bands.     -   4. To optimize the spacings of the cylindrical bands considering         the linearly-varying pressure profile imposed by the hydraulic         loads.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[13] FIG. 1 is an elevation view of the basic invention: a square footprint tank that utilizes cylinder sections for structural purposes.

FIG. 2 shows a cross-section of this same tank, and

FIG. 3 shows an isometric view.

FIG. 4 shows an isometric view of a rectangular footprint tank embodiment created by combining two square footprint tanks, each using the invention described herein. Such a tank may be extended indefinitely by combining additional square tank shapes.

FIG. 5 shows a plan view of the rectangular embodiment of FIG. 4.

FIG. 6 shows more production-friendly version of the basic invention, with chamfered corners and smooth transitions between cylinders and square prisms for more efficient use of material.

FIG. 7 shows an isometric view of a rectangular footrpint embodiment of the production-friendly design shown in FIG. 6.

In FIG. 8, the invention is further extrapolated to contain horizontal-axis cylinders between square footprint segments in order to provide additional hoops for strength.

FIG. 9 is a cross-sectional view of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

[14] FIG. 1 shows a plan view of the simplest tank disclosed here. When making a cross section of the tank at one of the cylinders, the result is shown in FIG. 2. The vertical walls of the tank 1 are formed of alternating vertically-extended squares and cylinders. The side lengths of the squares 2 and the diameters of the cylinders 3 are either identical or the cylinders 3 are slightly smaller than the squares 2, and the shapes are co-axial with cylinders perfectly set in the squares. Connecting the cylindrical and vertically-extended square elements are enclosing surfaces 4 necessary to close the tank. These surfaces 4 are near horizontal but are tapered, in molded tanks, to facilitate demolding. Surfaces 4 also transfer to the cylinders the hydraulic loads imposed on the planar vertical walls. The hoop strength of the cylinders is essential to the tank's structural integrity, but cylinder height should be minimized to maximize the liquid volume contained in the space occupied by the tank.

[15] FIG. 3 shows an isometric view of the tank 1 introduced in FIG. 1. The tank 1 includes three rectangular prisms 2, labeled 2 a, 2 b, and 2 c proceeding from bottom to top. The lowermost square-solid 2 a has flat horizontal bottom sheet 5 that forms a stable base for the tank 1. Proceeding upward, the square-solids 2 b and 2 c are increasingly taller in consideration that the hydraulic loads are diminishing and therefore the vertical wall spans may be increased without increasing tank wall deflections. Connecting the square-solid sections 2 a, 2 b, and 2 c are cylinders 3 a and 3 b. In a preferred design the structural loads transferred to each cylinder 3 are equal as a result of the variable vertical spacing of the cylinders, so the heights of the cylinders may be equal. The tank 1 also has a planar horizontal top surface 6 from which extends upward an optional access port 7. The typical near-horizontal transition surfaces 4 join the square solids and the cylinders to complete the tank enclosure. The surfaces 4 need not be perfectly horizontal; in fact, tapering the lower one slightly downward and the upper one slightly upward (in outward direction from the cylinder) has the advantage of providing “taper” that facilitates removal of the finished part from its mold. FIG. 3 also shows at its lowest cylinder 3 a that a high strength band 8 of metal, fiberglass, or other high tensile strength material may be placed around the molded polymeric cylinder 3 a (or 3 b or “3n”) to supplement the tensile strength of the molded polymeric material. Tank edges 9 are shown as sharp edges. For molding, structural, and material optimization reasons, tank edges 9 may have chamfers or fillets.

[16] FIG. 4 shows the isometric view of a “multi-square” tank 10 that is rectangular in plan and is ideal for placement against the wall of a building, particularly in residential garages or side yards, or for larger volume applications that must still be able to pass through doorways, similar to the referenced “doorway tanks.” The multi-square tank 10 uses the same structural strategy as the “single-square” tank 1 shown in FIGS. 1 through 3, using cylinders for their hoop strength in combination with the rectangular solid that allow tank volume to be maximized. Vertical surfaces 11 a, 11 b, and 11 c run continuously across the face of the tank, separated by the horizontal or nearly-horizontal surfaces 4 of the adjacent cylinder pairs 3 a and 3 b. The diameters of cylinder pairs 3 a and 3 b are slightly less than the side length of their super-scribed squares. Thus the tank 10 emerges from its mold with through holes 12 a and 12 b shown in FIG. 5 and continuous cylindrical bands 3 a and 3 b. The width of this “double-square” tank is twice its “into-the-page” dimension, and the width of a triple-square tank would be three times its into-the page dimension. The through holes permit reinforcement with high tensile bands (not shown) as shown in FIG. 3 for the single-square tank. Longer tanks minimize manifolding in indoor applications requiring thousands of gallons of storage.

[17] FIGS. 6 and 7 show isometric views similar to FIGS. 3 and 4 of alternative embodiments of this invention (with identical numbering) that feature chamfered edges 13 and smoothed corners 14 in place of sharp edges 9 and near-horizontal transition surfaces 4. The flat chamfer surfaces 13 slightly decrease the tank storage volume but may increase strength by limiting stress concentrations. Another embodiment of this invention replaces the chamfered vertical surfaces 13 with large-radius fillets.

[18] FIG. 8 shows an optional multi-square tank similar to FIGS. 4 and 5 but with the addition of a horizontal cylinder 15 where the rectangular solids are joined. This horizontal cylinder allows the use of high tensile bands (not shown) for further structural reinforcement. Partial cylinders 16 a and 16 b do not take full advantage of the hoop strength principle but provide additional structural geometry and allow the use of modified reinforcing bands fabricated from semi-circles and rigid planes (not shown). FIG. 9 shows a cross-section of this alternative embodiment through the horizontal cylinders and semi-cylinders.

[19] The number and spacing of cylindrical bands affect the required wall thickness to withstand the hydraulic loads. Increasing the number of bands can shorten the vertical spans, which will reduce deflections for a given wall thickness, or can allow thinner walls for an allowed deflection. The vertical spacings can be varied from closer at the bottom to wider at the top to maintain constant deflections in all flat sections. Table 1 tabulates “equal deflection” spacings for tanks with 1 through 6 cylinders (2 through 7 vertical wall sections). The spacings are shown in Table 1 as percentages of total tank water depth. For example, a “4 section” tank with three cylinders would have the following height percentages per section, moving from bottom to top: 20.8%, 22.3%, 24.8%, 32.0%. The economics of cylinder spacing can be studied for a given allowable deflection by computing the material requirements. Appropriately spaced, more cylinders allow thinner walls but require additional surface area in the square-to-cylinder transitions. And each added ring reduces storage volume. Adding high-tensile bands around the cylinders can also affect economics since either the strength of the cylinders or the deflection of the flat panels can become the limiting structural criterion.

[20] The number and spacing of cylindrical bands affect the required wall thickness to withstand the hydraulic loads. Increasing the number of bands can shorten the vertical spans, which will reduce deflections for a given wall thickness, or can allow thinner walls for an allowed deflection. The vertical spacings can be varied from closer at the bottom to wider at the top to maintain constant deflections in all flat sections. FIG. 8 tabulates “equal deflection” spacings for tanks with 1 through 6 cylinders (2 through 7 vertical wall sections). The spacings are shown in FIG. 8 as percentages of total tank water depth. For example, a “4 section” tank with three cylinders would have the following height percentages per section, moving from bottom to top: 20.8%, 22.3%, 24.8%, 32.0%. The economics of cylinder spacing can be studied for a given allowable deflection by computing the material requirements. More cylinders, appropriately spaced, allow thinner walls but require additional surface area in the square-to-cylinder transitions. And, each added ring reduces storage volume. Adding high-tensile bands around the cylinders can also affect economics since either the strength of the cylinders or the deflection of the flat panels can become the limiting structural criterion.

TABLE 1 PERCENTAGE OF TOTAL HEIGH BY SECTION (H1 = bottom section) Tank Profile (# of sections) Section 2 3 4 5 6 7 Height section section section section section section H1 43.7% 28.2% 20.8% 16.5% 13.7% 11.7% H2 56.3% 31.4% 22.3% 17.4% 14.3% 12.1% H3 NA 40.5% 24.8% 18.6% 15.0% 12.6% H4 NA NA 32.0% 20.7% 16.1% 13.2% H5 NA NA NA 26.7% 17.9% 14.2% H6 NA NA NA NA 23.1% 15.8% H7 NA NA NA NA NA 20.4%

[21] With planar panels 2 and 11 and wall thickness comparable to the cited “doorway” tanks, the designs disclosed here lose less than 3% of the potential “rectangular solid” volume as a result of their cylindrical bands, compared with 10% lost volume for the available tanks. 

1. A molded liquid storage tank whose vertical walls are formed by alternating, vertically-stacked and aligned cylinders and rectangular prisms with square footprints, with connecting horizontal planes enclosing the spaces between.
 2. Claim 1 with the cylinders spaced more closely near the tank bottom and more widely near the tank top to create approximately equal deflections of vertical wall sections under hydraulic tank loads.
 3. Claim 1 with the cylinders spaced more closely near the tank top and more widely near the tank bottom to create approximately equal deflections of vertical wall sections in stratified thermal storage applications where polymer materials are less stiff at the top of the tank than at the bottom due to higher temperature.
 4. Claim 1 with equally-spaced cylinders.
 5. Claim 1 with one or more cylinders reinforced with external straps.
 6. Claim 1 with the edges of each vertically-extended square chamfered.
 7. Claim 1 with the edges of each vertically-extended square formed to a radius.
 8. Claim 1 with connecting planes flared from horizontal to facilitate demolding.
 9. Claim 1 with smoothed corners to reduce stress concentrations and optimize material usage.
 10. A molded liquid storage tank whose vertical walls are formed by alternating, vertically-stacked and aligned cylinders and rectangular prisms with rectangular footprints, with at least two side-by-side vertical-axis cylinders, with the rectangle footprint aspect ratio having integer values (2:1, 3:1, etc.) corresponding to the number of cylinders, and with connecting horizontal planes enclosing the spaces between the cylinders and the rectangles.
 11. Claim 10 with cylinder diameter slightly smaller than the length of the shorter side of the rectangle, resulting in through-holes where the at least two adjacent cylinders meet.
 12. Claim 10 with the cylinders spaced more closely near the tank bottom and more widely near the tank top.
 13. Claim 10 with cylinders variably spaced to create approximately equal deflections of vertical wall sections under hydraulic tank loads.
 14. Claim 10 with equally-spaced cylinders.
 15. Claim 10 with one or more cylinders reinforced with external straps.
 16. Claim 10 with the edges of each rectangular prism chamfered.
 17. Claim 10 with the edges of each rectangular prism formed to a radius.
 18. Claim 10 with corners smoothed to reduce stress concentrations.
 19. Claim 10 with connecting planes flared from horizontal to facilitate demolding.
 20. Claim 10 with the rectangular tank footprint broken up by horizontal-axis cylinders spaced between the adjacent vertical-axis cylinders. 